Dust on Snow Controls Springtime River Rise in West

A coating of dust on snow speeds the pace of snowmelt in the spring. Credit: NASA › Larger view

A new study has
found that dust, not spring warmth, controls the pace of spring snowmelt that
feeds the headwaters of the Colorado River. Contrary to conventional wisdom, the
amount of dust on the mountain snowpack controls how fast the Colorado Basin's rivers
rise in the spring regardless of air temperature, with more dust correlated
with faster spring runoff and higher peak flows.

The finding is
valuable for western water managers and advances our understanding of how
freshwater resources, in the form of snow and ice, will respond to warming
temperatures in the future. By improving knowledge of what controls the melting
of snow, it improves understanding of the controls on how much solar heat Earth
reflects back into space and how much it absorbs -- an important factor in
studies of weather and climate.

When snow gets
covered by a layer of windblown dust or soot, the dark topcoat increases the
amount of heat the snow absorbs from sunlight. Tom Painter of NASA's Jet
Propulsion Laboratory in Pasadena, California, has been researching the
consequences of dust on snowmelt worldwide. This is the first study to focus on
which has a stronger influence on spring runoff: warmer air temperatures or a
coating of dust on the snow.

Windblown dust has
increased in the U.S. Southwest as a result of changing climate patterns and
human land-use decisions. With rainfall decreasing and more disturbances of the
land, protective crusts on soil are removed and more bare soil is exposed.
Winter and spring winds pick up the dusty soil and drop it on the Colorado
Rockies to the northeast. Historical lake sediment analyses show there is
currently an annual average of five to seven times more dust falling on the Rocky
Mountain snowpack than there was before the mid-1800s.

Painter and
colleagues looked at data on air temperature and dust in a mountain basin in
southwestern Colorado from 2005 to 2014, and streamflow from three major tributary
rivers that carry snowmelt from these mountains to the Colorado River. The
Colorado River's basin spans about 246,000 square miles (637,000 square
kilometers) in parts of seven western states.

The researchers
found that the effects of dust dominated the pace of the spring runoff even in
years with unusually warm spring air temperatures. Conversely, there was almost
no statistical correlation between air temperature and the pace of runoff.

"We found
that when it's clean, the rise to the peak streamflow is slower, and generally
you get a smaller peak." Painter said. "When the snowpack is really
dusty, water just blasts out of the mountains." The finding runs contrary
to the widely held assumption that spring air temperature determines the
likelihood of flooding.

Coauthor McKenzie Skiles, an assistant professor in the University
of Utah Department of Geography, said that while the impacts of dust in the air,
such as reduced air quality, are well known, the impacts of the dust once it's
been deposited on the land surface are not as well understood. "Given the
reliance of the western U.S. on the natural snow reservoir, and the Colorado
River in particular, it is critical to evaluate the impact of increasing dust
deposition on the mountain snowpack," she said.

Painter pointed
out that the new finding doesn't mean air temperatures in the region can be
ignored in considering streamflows and flooding, especially in the future.
"As air temperature continues to climb, it's going to have more
influence," he said. Temperature controls whether precipitation falls as
snow or as rain, for example, so ultimately it controls how much snow there is
to melt. But, he said, "temperature is unlikely to control the variability
in snowmelt rates. That will still be controlled by how dirty or clean the
snowpack is."

Skiles noted, "Dust on snow does not only impact the mountains that make
up the headwaters of Colorado River. Surface darkening has been observed in
mountain ranges all over the world, including the Alps and the Himalaya. What
we learn about the role of dust deposition for snowmelt timing and intensity
here in the western U.S. has global implications for improved snowmelt
forecasting and management of snow water resources."

The study,
titled "Variation in
rising limb of Colorado River snowmelt runoff hydrograph controlled by dust
radiative forcing in snow," was published today in the journal Geophysical
Research Letters. Coauthors are from the University of Utah, Salt Lake City; University
of Colorado, Boulder; and University of California, Santa Barbara.